1. Field of the Invention
The present invention relates to improvements of an antenna suitably used for a radio-frequency identification tag capable of writing and reading information in a non-contact fashion.
2. Description of Related Art
There is known an RFID (Radio-Frequency Identification) communication system wherein a radio-frequency identification tag communication device (interrogator) reads out information, in a non-contact fashion, from small-sized radio-frequency identification tags (transponders) on which desired information is written. In this RFID communication system, the radio-frequency identification tag communication device is capable of reading out the information from the radio-frequency identification tags, even where the radio-frequency identification tags are contaminated or located at positions invisible from the radio-frequency identification tag communication device. For this reason, the RFID communication system is expected to be used in various fields, such as management and inspection of articles of commodity.
One of fundamental needs to be satisfied regarding the RFID communication system is to reduce the size of the radio-frequency identification tags. To reduce the size of the radio-frequency identification tags, it is particularly required to accommodate an antenna of each radio-frequency identification tag in a surface area as small as possible, while maintaining characteristics of the antenna desired for radio-frequency transmission and reception of information. An example of a structure of the antenna takes the form of a planar or flat meander line structure. JP-2004-228797A (corresponding to U.S. Pat. No. 7,109,945 B2) discloses an example of a planar antenna for television reception. This planar antenna has a planar meander line structure which includes line conductors formed in a meandering or zigzag pattern so that the antenna can be accommodated in a surface area as small as possible, while maintaining the desired characteristics such as a longitudinal dimension.
However, the size reduction of the radio-frequency identification tag has a problem specific to its construction. Namely, the size reduction of the radio-frequency identification tag results in reduction of an input impedance of its antenna, and an increase of a degree of mismatch between the input impedance of the antenna and an input impedance of an IC circuit portion connected to the antenna, so that there is a risk of deterioration of the characteristics of the antenna such as its sensitivity value and communication distance. Therefore, there have been a need for developing a small-sized antenna which has a good impedance match with the IC circuit portion and which maintains desired communication characteristics, and a need for developing a radio-frequency identification tag provided with such a small-sized antenna.